More than two million people in the United States alone are incapacitated by chronic pain on any given day (T. M. Jessell & D. D. Kelly, Pain and Analgesia in PRINCIPLES OF NEURAL SCIENCE, third edition (E. R. Kandel, J. H. Schwartz, T. M. Jessell, ed., (1991)). Unfortunately, current treatments for pain are only partially effective, and many also cause life-style altering, debilitating, and/or dangerous side effects. For example, non-steroidal anti-inflammatory drugs (“NSAIDs”) such as aspirin, ibuprofen, and indomethacin are moderately effective against inflammatory pain but they are also renally toxic, and high doses tend to cause gastrointestinal irritation, ulceration, bleeding, increased cardiovascular risk, and confusion. Patients treated with opioids frequently experience confusion and constipation, and long-term opioid use is associated with tolerance and dependence. Local anesthetics such as lidocaine and mixelitine simultaneously inhibit pain and cause loss of normal sensation. In addition, when used systemically local anesthetics are associated with adverse cardiovascular effects. Thus, there is currently an unmet need in the treatment of chronic pain.
Pain is a perception based on signals received from the environment and transmitted and interpreted by the nervous system (for review, see Millan, M. J., The induction of pain: an integrative review. Prog Neurobiol 57:1-164 (1999)). Noxious stimuli such as heat and touch cause specialized sensory receptors in the skin to send signals to the central nervous system (“CNS”). This process is called nociception, and the peripheral sensory neurons that mediate it are nociceptors. Depending on the strength of the signal from the nociceptor(s) and the abstraction and elaboration of that signal by the CNS, a person may or may not experience a noxious stimulus as painful. When one's perception of pain is properly calibrated to the intensity of the stimulus, pain serves its intended protective function. However, certain types of tissue damage cause a phenomenon, known as hyperalgesia or pronociception, in which relatively innocuous stimuli are perceived as intensely painful because the person's pain thresholds have been lowered. Both inflammation and nerve damage can induce hyperalgesia. Thus, persons afflicted with inflammatory conditions, such as sunburn, osteoarthritis, colitis, carditis, dermatitis, myositis, neuritis, inflammatory bowel disease, collagen vascular diseases (which include rheumatoid arthritis and lupus) and the like, often experience enhanced sensations of pain. Similarly, trauma, surgery, amputation, abscess, causalgia, collagen vascular diseases, demyelinating diseases, trigeminal neuralgia, cancer, chronic alcoholism, stroke, thalamic pain syndrome, diabetes, herpes infections, acquired immune deficiency syndrome (“AIDS”), toxins and chemotherapy cause nerve injuries that result in excessive pain.
As the mechanisms by which nociceptors transduce external signals under normal and hyperalgesic conditions become better understood, processes implicated in hyperalgesia can be targeted to inhibit the lowering of the pain threshold and thereby lessen the amount of pain experienced.
Bradykinin (BK) and the related peptide, kallidin (Lys-BK) (see Table 3) mediate the physiological actions of kinins on the cardiovascular and renal systems. However, the active peptides, BK and kallidin, are quickly degraded by peptidases in the plasma and other biological fluids and by those released from a variety of cells, so that the half-life of BK in plasma is reported to be approximately 17 seconds (1). BK and kallidin are rapidly metabolized in the body by carboxypeptidase N, which removes the carboxyterminal arginine residue to generate des-Arg BK or des-Arg kallidin. Des-Arg-kallidin is among the predominant kinins in man and mediate the pathphysiological actions of kinins in man. In addition to being a very potent proinflammatory peptide, des-Arg-BK or des-Arg-kallidin is known to induce vasodilation, vascular permeability, and bronchoconstriction (for review, see Regoli and Barabe, Pharmacology of Bradykinin and Related Kinins, Pharmacological Reviews, 32(1):1-46 (1980)). In addition, des-Arg-BK and des-Arg-kallidin appear to be particularly important mediators of inflammation and inflammatory pain as well as being involved in the maintenance thereof. There is also a considerable body of evidence implicating the overproduction of des-Arg-kallidin in conditions in which pain is a prominent feature such as septic shock, arthritis, angina, and migraine.
The membrane receptors that mediate the pleiotropic actions of kinins are of two distinct classes, designated B1 and B2. Both classes of receptors have been cloned and sequenced from a variety of species, including man (Menke, et al, Expression cloning of a human b1 bradykinin receptor. J. Biol. Chem. 269:21583-21586 (1994); Hess et al, Cloning and pharmacological characterization of a human bradykinin (BK-2) receptor. Biochem. Biophys. Res. Commun. 184, 260-268 (1992)). They are typical G protein coupled receptors having seven putative membrane spanning regions. In various tissues, BK receptors are coupled to every known second messenger. B2 receptors, which have a higher affinity for BK, appear to be the most prevalent form of bradykinin receptor. Essentially all normal physiological responses and many pathophysio-logical responses to bradykinin are mediated by B2 receptors.
B1 receptors, on the other hand, have a higher affinity for des-Arg-BK (see Table 3) compared with BK, whereas des-Arg-BK is inactive at B2 receptors. In addition, B1 receptors are not normally expressed in most tissues. Their expression is induced upon injury or tissue damage as well as in certain kinds of chronic inflammation or systemic insult (Marceau, F., et al., Kinin B1 receptors: a review. Immunpharmacology, 30:1-26 (1995)). Furthermore, responses mediated by B1 receptors are up-regulated from a null level following administration of bacterial lipopolysaccharide (LPS) or inflammatory cytokines in rabbits, rats, and pigs (Marceau et al., (1998)).
The pain-inducing properties of kinins coupled with the inducible expression of B1 receptors make the B1 receptor an interesting target in the development of anti-inflammatory, antinociceptive, antihyperalgesic and analgesic agents that may be directed specifically at injured tissues with minimal actions in normal tissues. While a variety of peptide antagonists targeting the B1 receptor have been identified, their development as therapeutic analgesics has been stymied by poor efficacious half-lives resulting from very rapid degradation by tissue and serum peptidases and efficient renal clearance. More recently, peptide analogs having non-natural amino acid substituents have been shown to be resistant to peptidases in in vitro stability assays (for review, see Regoli et al, Bradykinin receptors and their antagonists. European Journal of Pharmacology, 348:1-10 (1998); Stewart, J. M., et al, Bradykinin antagonists: present progress and future prospects. Immunopharmacology, 43:155-161 (1999); and Stewart, J. M., et al., Metabolism-Resistant Bradykinin Antagonists: Development and Applications. Biol. Chem., 382:37-41 (2001)).
Covalent conjugation of proteins with poly(ethylene glycol) (PEG) has been widely recognized as an approach to significantly extend the in vivo circulating half-lives of therapeutic proteins. PEGylation achieves this effect predominately by retarding renal clearance, since the PEG moiety adds considerable hydrodynamic radius to the protein (Zalipsky, S., et al., Use of functionalized poly(ethylene glycol)s for modification of polypeptides., in Poly(ethylene glycol) chemistry: Biotechnical and biomedical applications., J. M. Harris, Editor. (1992), Plenum Press: New York. p. 347-370.). Additional benefits often conferred by PEGylation of proteins include increased solubility, resistance to proteolytic degradation, and reduced immunogenicity of the therapeutic polypeptide. The merits of protein PEGylation are evidenced by the commercialization of several PEGylated proteins including PEG-Adenosine deaminase (Adagen™/Enzon Corp.), PEG-L-asparaginase (Oncaspar™/Enzon Corp.), PEG-Interferon α-2b (PEG-Intron™/Schering/Enzon), PEG-Interferon α-2a (PEGASYS™/Roche) and PEG-G-CSF (Neulasta™/Amgen) as well as many others in clinical trials. PEGylation of small therapeutic peptides, on the other hand, presents unique challenges and has not been broadly applied. One of the greatest obstacles to peptide PEGylation is the essential requirement that biological activity be preserved in the final conjugate. Because therapeutic peptides often comprise the minimal sequence required for activity and are therefore very small, they are relatively intolerant to substitution. PEG moieties are disproportionately larger than the peptide itself and consequently are more likely to interfere sterically with specific peptide:receptor binding interactions required for activity. Thus, a peptide's ability to tolerate PEGylation and still retain sufficient specific activity to be a useful therapeutic is quite unpredictable and must be empirically determined (Morpurogo, et al., Selective Alkylation and Acylation of α and ε Amino Groups with PEG in a Somatostatin Analogue: Tailored Chemistry for Optimized Bioconjugates. Bioconjugate Chem. 13:1238-1243 (2002)).
Clearly, there is a need for new, safe and effective treatments for inflammation and pain. It would be an advantage to have a B1 specific peptide antagonist that is better able to tolerate systemic exposure during treatment, by enhancing the circulating life (delayed clearance), solubility, stability, and/or decreasing the immunogenicity of the molecule. Increased circulating life would result in a less frequent dosing regimen and a less frequent dosing schedule would be more convenient to both physicians and patients, and would be particularly helpful to those patients involved in self-administration. Other advantages to less frequent dosing may include less drug being introduced into patients and increased compliance.